The ability of tiered-therapy implantable cardioverter defibrillators (ICDs) to deliver appropriate therapy depends in part on their ability to discriminate among various cardiac arrhythmias. The primary parameter used for discrimination has been heart rate. Other characteristics of electrograms which have been used are suddenness of onset of a high rate, rate stability, and morphology of the QRS complex. The limited computational capacity of contemporary devices has, for morphology-based discriminators, led developers to focus on techniques that rely on rather superficial properties of the electrogram. It is desirable to develop an algorithm which operates on the fundamental properties of the electrogram, but is not computationally intensive.
An arrhythmia of ventricular origin is conducted throughout the ventricles by a different path than the specialized conduction system which conveys supraventricular rhythms. As used herein, supraventricular rhythms include both normal rhythms such as sinus rhythm as well as arrhythmias of supraventricular origin such as atrial fibrillation. The different pathways used by ventricular and supraventricular rhythms have different dynamics, which give rise to differences in morphology between these two classes of rhythms. It would be beneficial to have an arrhythmia discrimination algorithm which treats electrograms as having arisen from a dynamical system.
A dynamical system is a system that can be described by a set of coupled differential equations. The degrees of freedom, m, of the system is the number of variables needed to characterize the system's behavior, or equivalently, the number of equations in the set of coupled, first-order differential equations. A phase space is a mathematical m-dimensional space where each dimension is associated with one of the m system variables. Since the state of the system at a particular time is given by the value of each variable, the state can be represented by the location of a point in the phase space. As the system evolves with time, the value of each variable changes, and the point characterizing the state of the system moves in the space. The time-evolution of the system is thus characterized by the trajectory of the point in phase space. The trajectory is determined by the differential equations governing the system, so the phase space representation embodies the dynamical properties of the equations.
In analyzing experimental data, one typically has access to a single variable, such as position, rather than the entire set of m state variables. One of the profound insights that has arisen from chaos theory is the recognition that a topologically equivalent representation of phase space can be constructed from the observation of a single variable. Access to all the state variables is therefore unnecessary.
It is reasonable to view the voltage recorded by an electrogram as arising from a dynamical system. Different systems are responsible for propagation through the myocardium and the specialized conducting system. Differences between the trajectories of rhythms of ventricular and supraventricular origin should then be apparent in the reconstructed phase space. An algorithm based on this approach is thus more than just an empirical technique: it relies on the differences in the fundamental dynamics of the system, the same information given by the differential equations that model the system. However, since our interest is in discrimination, and not in identifying the intrinsic properties of the putative dynamical systems, the necessary degree of rigor is greatly relaxed. For example, it is not necessary to determine the true dimensionality of the system or address issues of noise; it is sufficient to simply extract enough of the dynamical differences that the systems can be distinguished.
It is therefore an object of the present invention to provide a method and apparatus for distinguishing between cardiac rhythms of supraventricular and ventricular origins.
It is a further object of the invention to provide a computationally efficient method for using depolarization morphology for distinguishing cardiac rhythms.